I was going to say " £30?" but I've just seen that it looks to be a decent amount longer and more awesome than the first one, so it could be worth it.
Portal 2 pre order is available.
You can save 10% by pre ordering it now for £26.99
http://store.steampowered.com/app/620/
How can Valve ask £26.99?
I pre ordered the Orange Box for around that price (prices were in US Dollars at the time) from Steam and got 5 games.
If they reckon it's 6 hours game time for SP and another 6 for CO OP then it just aint worth £26.99
I'll wait a while until its on offer.
I was expecting them to at least offer a 4 pack deal for £20 each, like L4D2 .